Consider an apparatus in which an object moves over a two-dimensional surface within a predetermined area in response to an external force applied to the object. The object will be referred to as a puck in the following discussion, and the area over which it is permitted to move will be referred to as the puck field of motion. For example, a computer pointing device that is analogous to a “mouse” is described in a co-pending patent application. This device includes a puck that moves over a surface within a puck field of motion in response to a user applying force to the puck via a finger. The surface has position sensors that sense the position of the puck within the puck field of motion and report that position to a data processing system coupled to the device. When the user releases the puck, the puck returns to a predetermined location within the puck field of motion. The restoring force for returning the puck is generated by springs that connect the puck to a support at the edge of the puck field of motion.
A similar mechanism can be utilized to construct an accelerometer. In this case, the puck moves when the device carrying the puck accelerates. The acceleration can be quantified by measuring the puck position as a function of time. Once the acceleration has ceased, the puck is returned to its starting position by the springs.
In the case of a mouse, the springs used to restore the puck position should provide a restoring force that recenters the puck without requiring that the user apply a force to move the puck that is large enough to cause the user's hand to become fatigued. In addition, the force the user applies to move the puck should not vary over the puck field of motion, since such variations can interfere with the precision with which the user can position the puck. In addition, embodiments that are designed for use in laptop and handheld computers place a premium on both the lateral size of the pointing device and the thickness of the pointing device. Hence, designs in which the springs increase the thickness or lateral dimensions of the pointing device are not preferred. In the case of an accelerometer or the like, the restoring force should be uniform independent of the direction in which the puck moves.